The present invention relates to radiation detectors, particularly to semiconductor radiation detectors, and more particularly to wafer-fused semiconductor radiation detectors wherein an internal electrically conductive grid is located between ends of a pair of fused semiconductor pieces, and to a method for fabricating same.
Various types of radiation detectors have been developed for detecting gamma-rays and x-rays, among which are the planar semiconductor radiation detectors. Semiconductor radiation detectors generally operate by absorbing a quantum of gamma-ray or x-ray radiation and by converting the radiation energy into a number of electron-hole pairs that is proportional to the absorbed energy. After the conversion, the motion of the electrons and holes induce electrical signals on the detector electrodes. The electrical signals are also proportional to the energy of the absorbed radiation. Hence, by using a semiconductor radiation detector, one can detect gamma-ray and x-ray radiation and measure its energy spectrum.
The conventional planar semiconductor radiation detector is basically composed of a semiconductor material having a cathode on one surface and an anode on the opposite surface. A positive bias voltage is applied to the anode and a negative bias voltage is applied to the cathode. As x-rays or gamma-rays pass through the semiconductor material, electrons and holes are formed, and electrical signals are generated, and thus the energy of x-ray or gamma-ray radiation on the semiconductor material can be measured.
The conventional planar semiconductor radiation detector does not function well due to the poor electrical transport properties of the holes. Many of the common radiation detectors are made from CdZnTe or GaAs, with a cathode and anode made, for example, of gold, and for these semiconductors, the electrical signal due to the holes is typically much smaller than the electrical signal due to the electrons. These effects are due to the slower motion of the holes and greater probability of trapping of the holes within these materials. Because the total electrical signal is a sum of the signal due to the electrons and the holes, the signal due to the holes complicates the signal analysis and results in poor energy resolution and low efficiency for the detector.
The planar semiconductor radiation detector also suffers from a position dependence on the signal. For example, a signal due to electrons originating from radiation absorbed near the cathode will be larger than a signal originating from near the anode. Thus, the conventional planar semiconductor radiation detectors suffer from both poor electrical transport properties of the holes and from a position dependence of the signal.
Recent efforts have been directed to improve the energy resolution of the planar semiconductor radiation detectors and also to lessen the dependence of the signal on the position of the radiation absorption, and thus allow one to isolate the electrical signal due to the motion of electrons. These improved approaches are referred to as xe2x80x9celectron-only devicesxe2x80x9d and have shown to give superior energy resolution for x-ray and gamma-ray radiation over the conventional planar semiconductor radiation detectors. The xe2x80x9celectron-only devicesxe2x80x9d are exemplified by P. N. Luke, xe2x80x9cSingle-polarity charge sensing in ionization detectors using coplanar electrodes,xe2x80x9d Appl. Phys. Lett. 65 (22), Nov. 28, 1994; E. Y. Lee, et al., xe2x80x9cDevice Simulation of an Unipolar Gamma-Ray Detector,xe2x80x9d Mat. Res. Soc. Symp. Proc., 487, p. 537 (1998), U.S. Pat. No. 5,677,539, issued Oct. 14, 1997 to B. Apotovsky, et al., U.S. application Ser. No. 09/075,419 filed May 8, 1998, entitled, xe2x80x9cMethod and Apparatus for Electron-Only Radiation Detectors from Semiconductor Materialsxe2x80x9d by Lund, et al., now U.S. Pat. No. 6,069,360, and U.S. application Ser. No. 09/075,351 filed May 8, 1998, entitled, xe2x80x9cHigh Resolution Ionization Detector and Array of Such Detectorsxe2x80x9d by McGregor, et al. now U.S. Pat. No. 6,175,120. These xe2x80x9celectron-only devicesxe2x80x9d place a third metallic electrode, called a grid, on the surface of the detector near the anode to electrostatically shield the anode from the signal originating between the grid and the cathode. In these devices, all the signals from the anode originates from a motion of the electrons and holes moving between the anode and the grid. Since the electrons move toward the anode while the holes move away from the anode toward the cathode, due to their polarities, the signal on the anode will be dominated by the motion of the electrons. Furthermore, the signal will have much less position dependence, since electron trapping between the grid and anode will be unlikely.
In an xe2x80x9celectron-only devicexe2x80x9d one can characterize the space between the grid and the cathode as a detection volume and the region between the grid and the anode as the measurement volume. Ideally, all radiation absorbed in the detection volume would give rise to electrical signals due only to the motion of the electrons in the measurement volume. However, there are several imperfections associated with the prior art of the xe2x80x9celectron-onlyxe2x80x9d detector, which are:
1. For the grid to shield the anode, the grid can not be placed too close to the anode. This decreases the measurement volume of the detector and therefore the radiation detection efficiency of the detector.
2. Many of the electrons created between the grid and the cathode are collected by the grid and produce no signal on the anode. Hence, these detectors have dead regions where no signals can be detected, leading to a loss of detector efficiency.
3. The internal electric field of the detector is highly non-uniform, due to the placement of the external grid. The electric field is uniform only very close to the cathode and the anode. The non-uniformity of the electric field causes variation in the charge collection time of the electrons. Since electron trapping does occur in the detector, this non-uniformity in the electric field results in variation of the signal strength with the position of the x-ray and gamma-ray absorption event, and hence in loss of the energy resolution. Attempts at correcting for the electron trapping by trying to deduce the position of the original radiation absorption are difficult due to the non-uniform internal electric field. This is commonly attempted by monitoring of the cathode signal and using it to correct the anode signal with electronic circuits external to the detector.
Another type of prior art radiation detectors utilize a grid called the Frisch grid. This type of detector contains a gas at a high pressure, and the Frisch grid comprises a metal mesh located between the cathode and the anode. The Frisch grid gives electron-only behavior and it has been the inspiration for a new class of electron-only detectors based on semiconductor materials. However, obviously one can not place a metal mesh through a solid semiconductor and hence it is not possible to directly implement the idea of the Frisch grid for semiconductor radiation detectors. Also, it is not possible to grow a metal mesh and then cover it up with a good quality semiconductor, since the resulting semiconductor overlayer always has poor electrical characteristics and low transmission through the interface, due to difficulties in the growth process. Simply pressing a metal mesh between two semiconductors does not make them a single piece, because in this device, electrons must cross the interface without becoming trapped by defects.
However, it has been recently discovered that, by applying high pressure and high temperature uniformly, it is possible to xe2x80x9cbondxe2x80x9d two clean semiconductor pieces or wafers together, without any glue, to form good interfaces. See Z. L. Leau, et al., Appl. Phys. Let. 56,737 (1990). This process is known as wafer bonding or wafer fusion. Wafer bonding has been successfully demonstrated in such semiconductor systems as GaAs/InP, GaN/InP, InGaAsP/Si, InP/SiO2/InP, LiTaO3/Si, Si/In(Sb), Si/SiO2, and LiTaO3/Si. See above referenced Z. L. Lain, et al.; Y. H. Lo, et al., Appl. Phys. Lett. 62,1038 (1993); A. R. Hawkins, et al., Appl. Phys. Lett. 68, 3692 (1996); R. K. Sink, et al., Appl. Phys. Lett., 68 (15), p. 2147 (1996); and F. A. Kish, et al., Electronic Letters 30, 1790 (1994).
By using wafer fusion, it is possible to put an electrically conductive grid between two semiconductors and bond them together to make the present invention, which is a wafer-fused semiconductor radiation detector (WAFUSRAD), and which is a direct analog of a Frisch grid gas detector, but using semiconductor materials and the wafer bonding technology. The WAFUSRAD of the present invention largely removes the above-referenced three imperfections of the previously known electron-only devices based on semiconductors, resulting in superior energy resolution and radiation detection efficiency. In addition, this detector has all the virtues of an electron-only device, exploiting the excellent transport properties of electrons over holes and having signals. That are independent of the position of interaction. Basically, the WAFUSRAD of the present invention utilizes an electrically conductive grid adjacent ends of two semiconductor pieces, with the electrical conductor grid located in grooves in the end of one of the semiconductor pieces.
It is an object of the present invention to provide an improved semiconductor radiation detector.
A further object of the invention is to provide a semiconductor radiation detector which substantially overcomes the imperfections of the known electron-only detectors.
A further object of the invention is to provide a semiconductor radiation detector that is a direct analog of a Frisch grid detector, but using wafer-fused semiconductor pieces with an electrically conductive grid formed in one of the semiconductor pieces.
Another object of the invention is to provide a wafer-fused semiconductor radiation detector.
Another object of the invention is to provide a semiconductor radiation detector which incorporates an internal electrically conductive grid adjacent ends of a pair of wafer-fused semiconductor pieces.
Another object of the invention is to provide a semiconductor radiation detector wherein an electrically conductive grid is formed in grooves in one end of a semiconductor piece and that piece is wafer-fused to another semiconductor piece.
Another object of the invention is to provide an internal electrically conductive grid for a semiconductor radiation detector wherein the semiconductor material of the detector may consist of two pieces of the same or different materials, of the same or different thickness, and with the conductor grid composed of a metal, a variety of different metals, heavily doped semiconductors, superconductors, a conductive polymer, or any other electrical conductor, or combination thereof.
Another object of the invention is to provide a fabrication technique for producing a semiconductor radiation detector having an internal electrical conductor grid which functions as a Frisch grid.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The present invention employs wafer fusion to insert an electrically conductive grid between two semiconductor pieces, one having a cathode and the other having an anode. The thus formed wafer-fused semiconductor radiation detector has an internal electrical conductor grid, which functions like the commonly known Frisch grid radiation detector. The electrically conductive grid is formed, for example, by etching spaced grooves across an end of a semiconductor piece, partially filling the grooves with an electrical conductor, and bonding the grooved end of the semiconductor piece to another semiconductor piece using the wafer fusion technique. The electrical conductor grid may be composed of any suitable electrical conductor, as above, and the semiconductor pieces may be of the same or different materials, and may have the same or different thickness and/or width. The conductor should have an electrical conductivity high enough to prevent significant voltage drop across members of the grid. This is important since such a condition would likely necessitate attaching a number of external electrodes to the grid at different locations to ensure that all grid members maintain the same electrical potential.
The detector of the present invention substantially overcomes the imperfections of the prior known electron-only detectors but exploits all the virtues of an electron-only detector; namely, the excellent transport properties of electrons over holes, and the position independent signal. The detector of this invention is a direct analog of a Frisch grid detector, but using semiconductor material, and wafer bonding technology.